The following references show related arts:    (1) Inventor: Cyrus Bamji, Assignee: Canesta Inc., “CMOS-Compatible Three-dimensional image sensor”, U.S. Pat. No. 6,323,942 B1, Nov. 27, 2001;    (2) R. Lange, P. Seitz, A. Biber, and S. Lauxtermann, “Demodulation pixels in CCD and CMOS technologies for time-of-flight ranging”, Proceedings of SPIE, Vol. 3965, pp. 177-188, 2000;    (3) Ryohei Miyagawa, Takeo Kanade, “CCD-based range-finding sensor”, IEEE Trans. Electron Devices, vol. 44, No. 10, pp. 1648-1652 (1997);    (4) Range Imaging Device, Japanese Patent Application Laid-open No. 2001-281336; and    (5) Charge Coupled Device, Japanese Patent Application Laid-open No. 2003-51988.
According to the method of (1), light pulses are transmitted, and the waveform of received signal pulses is shaped, by detecting peaks of the received signal pulses so that a delay time can be digitally measured using high-speed pulses. Since the method of (1) cannot work well without a sufficient light intensity that facilitates generation of a pulse signal from the received signal, application fields of method of (1) are limited.
The architectures of the methods of (2) and (3) are similar to each other. In the method of (2), in which CCD and CMOS are merged into a single chip through an integrated fabrication process, with a high frequency modulated light of 20 MHz, by utilizing a charge transfer mechanism of the CCD, a characteristic such that the distribution ratio of charges into two nodes depends on the delay time of the modulated light, in synchronization with the modulated light, is utilized. Such integrated CCD-CMOS manufacturing procedure increases cost.
According to the method of (3), the structure of the CCD is utilized to alternately transfer charges, which are generated by a pulse-modulated light, to two nodes, and a characteristic of the resulting charge distribution ratio depending on the delay time of the modulated light is utilized. Use of such a CCD requires a special fabrication process. Furthermore, while only a one-dimensional sensor (i.e., line sensor) is disclosed, implementation of a two-dimensional sensor (area sensor) established only with CCDs may be difficult, considering that all of the pixels should be simultaneously driven at a high frequency.
According to the methods of (4) and (5), though no detailed structure is disclosed, a structure in which charges generated by a photodiode are transferred to floating diffusion layers via two transfer gates is employed. However, incomplete transfer of the charges to two floating diffusion layers results in an insufficient performance. Therefore, a complex fabrication process must be added to fabricate a CMOS structure, resulting in a high fabrication cost. Meanwhile, because integration of a parallel-driving circuit for driving pixels is impossible in the CCD architecture by itself, an integrated CCD/CMOS manufacturing procedure is required for the CCD architecture. In conclusion, low cost does not go with high performance.